A seawater desalination plant which employs a reverse-osmosis membrane method is composed mainly of a pretreatment system, a high-pressure pump, a reverse-osmosis membrane cartridge, and an energy recovery system. In the seawater desalination plant, the intake seawater is processed to have certain water qualities by the pretreatment system, and the pretreated seawater is delivered into the reverse-osmosis membrane cartridge under pressure by the high-pressure pump. Part of the high-pressure seawater in the reverse-osmosis membrane cartridge passes through the reverse-osmosis membrane against the osmotic pressure and is desalinated, and fresh water is taken out from the reverse-osmosis membrane cartridge. The remaining seawater becomes in a concentrated state of a high salinity and is discharged as a reject from the reverse-osmosis membrane cartridge. More than half of electric power expense which is the largest operational cost in the seawater desalination plant is consumed for pressurizing the seawater by the high-pressure pump. Therefore, there have been proposed various methods for effectively recovering pressure energy possessed by the high-pressure reject with a high salinity discharged from the reverse-osmosis membrane cartridge.
For example, there is an energy recovery turbine which recovers kinetic energy of a high-speed jet produced by a nozzle or the like from the high-pressure reject with a turbine, and uses the recovered energy to assist in the power of a motor for driving the high-pressure pump. This system recovers the pressure energy of the reject by converting the pressure energy into turbine power and by using the turbine power to drive an impeller of the high-pressure pump whereby the turbine power is reconverted into pressure energy of the seawater. FIG. 1 shows such system, which is referred to as conventional art A. In this system, the energy of the fluid is recovered through two energy conversion processes, i.e., the process of converting fluid energy into shaft power and the process of converting the shaft power into fluid energy. Thus, this system is problematic in that even if the efficiency of the turbine and the efficiency of the high-pressure pump are 90% each, the overall energy recovery efficiency becomes low because the overall energy recovery efficiency is expressed by multiplication of both efficiencies (90%×90%=81%). Further, because the pump impeller and the turbine runner are disposed coaxially with each other, it is difficult to operate the turbine under optimum conditions at all times while following a change in operating conditions, resulting in a further efficiency reduction.
As an energy recovery system to solve the above problems, there is a system that employs an isobaric energy recovery device for recovering energy by a positive-displacement piston pumping action in which low-pressure pretreated seawater is pressurized by directly being pushed and pulled with a high-pressure reject through a piston in a chamber. This system is characterized in that the pressure energy possessed by the reject is recovered as fluid energy in a single energy conversion process by the isobaric energy recovery device, and thus the overall energy recovery efficiency becomes high efficiency of 90 to 98%. The piston in the chamber may be a cylindrical object that provides a physical partition wall between the pretreated seawater and the reject, or may be a hypothetical fluid piston in the form of an interface between the pretreated seawater and the reject. In this system, it is necessary to install a booster pump downstream of the isobaric energy recovery device because the pretreated seawater is pressurized to compensate for a pressure loss caused in the system and to merge into a high-pressure line. FIG. 2 shows such system, which is referred to as conventional art B. In this system, it is necessary to take some measures including the use of a variable-speed electric motor to drive the booster pump and the use of inverter control because there is a need for pressure compensation while following a change in pressure loss in the system due to environmental changes such as a temperature and a salinity, or membrane scaling, and the like. Because the suction pressure of the booster pump is nearly as high as the discharge pressure of the high-pressure pump and becomes a high-pressure condition, the booster pump is disadvantageous in that a pump having a special seal structure is required. Furthermore, attention should be paid to the controlling of the booster pump under transient operating condition which undergoes a change in operation condition, at the time of startup or shutdown.
In order to reduce the desalination costs incurred in producing fresh water in the seawater desalination plant, it is effective to increase the recovery rate and extract as much fresh water as possible from the same amount of intake seawater. Thus, there has been proposed a two-stage reverse-osmosis membrane system in which the reject from the above reverse-osmosis membrane cartridge is processed further by a second reverse-osmosis membrane cartridge. Since the reject from the first reverse-osmosis membrane cartridge has a higher salinity than the intake seawater, a second high-pressure pump may be disposed upstream of the second reverse-osmosis membrane cartridge in order to overcome an increase in the osmotic pressure thereof, thereby further boosting the pressure of the reject from the first reverse-osmosis membrane cartridge. In this case also, the reject from the second reverse-osmosis membrane cartridge has high pressure energy. Therefore, it is important to recover the high pressure energy of the reject as pressure energy of the high-pressure seawater directed toward the first and second reverse-osmosis membrane cartridges, and to reduce energy consumption of the entire system, as is the case with the single-stage reverse-osmosis membrane system.
The conventional art will be described below in detail.
(Conventional Art A)
A seawater desalination plant which employs a reverse-osmosis membrane method will be taken up, and the problems of the conventional art will specifically be described.
As shown in FIG. 1, the seawater 1 that has been supplied to the system by a feed pump 2 is processed to have certain water qualities by a pretreatment system 3, and is then pressurized by a high-pressure pump 5 driven by an electric motor 6 and delivered into a reverse-osmosis membrane cartridge 8. Part of the seawater in a high-pressure chamber 9 of the reverse-osmosis membrane cartridge passes through a reverse-osmosis membrane 10 against the osmotic pressure and is desalinated, and desalinated water 12 is taken out from a low-pressure chamber 11. The remaining seawater becomes in a concentrated state of a high salinity and is discharged as a concentrated reject from the reverse-osmosis membrane cartridge 8 to a reject line 13. The pressure energy of the high-pressure reject discharged from the reverse-osmosis membrane cartridge 8 is recovered as shaft power by an energy recovery turbine 14 having a rotating runner. The recovered power contributes to reduction of the shaft driving power of the electric motor 6 which is coaxially coupled to the turbine runner. The reject from which the pressure energy has been removed by the operation of the turbine 14 is discarded from a discharge line 15 to the outside of the system.
(Conventional Art B)
The conventional art B that employs an isobaric energy recovery device will be described below with reference to FIG. 2. According to the conventional art B, the pretreated seawater is pressurized by a positive-displacement piston pumping action in which the high-pressure energy possessed by the reject from a reverse-osmosis membrane cartridge is used to push and pull pistons in a plurality of pressure exchange chambers, thereby pressurizing the pretreated seawater and then discharging the pressurized seawater successively from the chambers.
The seawater 1 that has been supplied to the system by a feed pump 2 is processed to have certain water qualities by a pretreatment system 3, and is then pressurized by a high-pressure pump 5 driven by an electric motor 6 and delivered via a high-pressure line 7 into a reverse-osmosis membrane cartridge 8. On the other hand, part of the seawater in a high-pressure chamber 9 of the reverse-osmosis membrane cartridge passes through a reverse-osmosis membrane 10 against the osmotic pressure and is desalinated, and then desalinated water 12 is taken out from a low-pressure chamber 11. The remaining seawater becomes in a concentrated state of a high salinity and is discharged as a concentrated reject from the reverse-osmosis membrane cartridge 8 to a reject line 13. The pressure energy of the high-pressure reject discharged from the cartridge is introduced into pressure exchange chambers 20 sequentially through a control valve 19, whereby the respective pistons in the chambers are moved to pressurize the pretreated seawater in the chambers 20. The reject in each of the chambers which has moved the piston is disconnected from the reject line 13 by the control valve 19, and is discarded as a low-pressure reject from the chamber 20 to the outside of the system via a discharge line 15 while the reject is replaced by the pretreated seawater supplied from the supply line 4 to the chamber 20. The pretreated seawater having a low pressure in the chamber 20 which has replaced the reject is pressurized by the high-pressure reject newly introduced into the chamber 20 by the control valve 19. In this manner, the above cycle is repeated. By the isobaric energy recovery device 21 having the above structure, part of the seawater in the supply line 4 is pumped up and is discharged to the discharge line 22, and finally merges into the high-pressure line 7 from the outlet of the high-pressure pump 5. However, the fluid in the discharge line 22 has a lower pressure than the fluid in the high-pressure line 7 due to a pressure loss of the reverse-osmosis membrane cartridge 8 and the piping, a loss in the control valve 19, and the like. Therefore, in order to allow these fluids to merge together, a booster pump 17 driven by a variable-speed electric motor 18 is provided between the discharge line 22 and the high-pressure line 7.